Characterization and simulation on antireflective coating of amorphous silicon oxide thin films with gradient refractive index

Accepted Manuscript Characterization and simulation on antireflective coating of amorphous silicon oxide thin films with gradient refractive index Lu Huang, Qi Jin, Xingling Qu, Jing Jin, Chaochao Jiang, Weiguang Yang, Linjun Wang, Weimin Shi PII:

S0749-6036(16)30239-7

DOI:

10.1016/j.spmi.2016.05.026

Reference:

YSPMI 4348

To appear in:

Superlattices and Microstructures

Received Date: 11 April 2016 Revised Date:

16 May 2016

Accepted Date: 19 May 2016

Please cite this article as: L. Huang, Q. Jin, X. Qu, J. Jin, C. Jiang, W. Yang, L. Wang, W. Shi, Characterization and simulation on antireflective coating of amorphous silicon oxide thin films with gradient refractive index, Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.05.026. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Characterization and simulation on antireflective coating of

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amorphous silicon oxide thin films with gradient refractive index

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Lu Huang

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Weiguang Yang a, Linjun Wang a, Weimin Shi a

, Qi Jin a, Xingling Qu a, Jing Jin a, Chaochao Jiang a,

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a,*

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a

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China

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School of Materials Science and Engineering, Shanghai University, Shanghai 200444,

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*corresponding author: Tel. numbers: +86 21 66138066, E-mail address: [email protected] (L. Huang)

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ABSTRACT

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The optical reflective properties of silicon oxide (SixOy) thin films with gradient

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refractive index are studied both theoretically and experimentally. The thin films are

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widely used in photovoltaic as antireflective coatings (ARCs). An effective finite

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difference time domain (FDTD) model is built to find the optimized reflection spectra

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corresponding to structure of SixOy ARCs with gradient refractive index. Based on the

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simulation analysis, it shows the variation of reflection spectra with gradient

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refractive index distribution. The gradient refractive index of SixOy ARCs can be

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obtained in adjustment of SiH4 to N2O ratio by plasma-enhanced chemical vapor

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deposition (PECVD) system. The optimized reflection spectra measured by

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UV-visible spectroscopy confirms to agree well with that simulated by FDTD method.

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Keywords: antireflective coating; amorphous silicon oxide; gradient refractive index;

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finite difference time domain

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1. INTRODUCTION

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In the recent years, amorphous silicon (α-Si) thin films on glass substrates have

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attracted a great deal of attention in photovoltaics, microelectronics and display

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technologies because of its potential applications for electronic devices, especially

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thin film solar cells [1-3]. Efficient solar cells must satisfy two kinds of requirements:

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both strongly optical absorption and effectively electrical transportation. To enhance

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performance of antireflection and passivation, dielectric oxide layers must be selected

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to deposit on the surface of α-Si thin films. The antireflective coatings (ARCs) have

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been widely used in manufacturing process of conventional crystalline solar cells

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(c-Si) to benefit optical absorption, such as silicon nitride (SixNy) deposited on the

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surface of n-type c-Si [4-6] and aluminum oxide (AlxOy) prepared on the surface of

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p-type c-Si [7-9]. Because of the match of lattice to α-Si substrates, amorphous silicon

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oxide (SixOy) thin films have been used to prepare ARCs by plasma-enhanced

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chemical vapor deposition (PECVD) [10-11], magnetron sputtering [12-13] and

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sol-gel methods [14-16]. Compared to physical vapor deposition (PVD) method, it is

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more suitable to prepare multilayer films with different refractive index by PECVD

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method. The average reflectance for double-layer ARCs are lower over a broader

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wavelength range than for a single-layer ARC, because single-layer ARC has only

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minimal point of reflectance [17-18]. With these requirements, double-layer SixOy

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ACCEPTED MANUSCRIPT ARCs with gradient refractive index not only decrease electrical recombination

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effectively, but also enhance optical antireflection strongly [19-20]. However, it is

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worth discussing how to enhance antireflective effect of α-Si thin film solar cells

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through different distribution structures of SixOy ARCs.

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In this paper, we present theoretical and experimental study of optical reflective

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properties of SixOy ARCs with gradient refractive index. The finite difference time

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domain (FDTD) method is used to study the light-modulated characteristics of ARCs

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with gradient refractive index. Reflection spectra for different distribution structures

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of SixOy ARCs with gradient refractive index are simulated by FDTD method. This

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experimental work aims to focus on the correlation between reflection spectra and

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gradient refractive index distribution. Surface characterization and FDTD simulation

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analysis of SixOy ARCs with gradient refractive index are investigated simultaneously.

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2. EXPERIMENTAL DETAILS

The hydrogenated amorphous silicon oxide (SixOy:H) thin films are prepared by

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the following procedure. First, SixOy:H films as a layer is deposited on the glass

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substrates using PECVD by silane (SiH4), nitrous oxide (N2O) and hydrogen (H2).

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The experimental parameters of deposited SixOy:H thin films are shown in Tab.1. The

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PECVD system is HLF-400 made in Beijing Beiyi Innovation Vacuum Technology

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Co. Ltd. Before the thin films are deposited on glass substrate, heat-resistant tape has

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been adhered to the corner of glass substrate. After we finish depositing SixOy:H thin

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films and uncover tape from the substrate, the thickness of thin film is measured on

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measurement of stylus profiler is Vecco Dektak 150. Then double-layer structures are

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formed on the surface of first layer according to above procedure again. All of

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samples have been dehydrogenated, after thin films were annealed in vacuum

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condition at 200°C for 30 minutes.

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The structural orientation of SixOy films are measured by a D/Max-2200 X-ray

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diffractometer (XRD) of Rigaku using the Cu Kα radiation. The cross-section

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morphology and element composition of thin films is characterized respectively by

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scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) made

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by JEOL JSM-6700F. The refractive index and thickness are measured by

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spectroscopic ellipsometer of Sentech SE-400adv. UV-visible reflection spectra are

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performed by Hitachi U-2910 spectrophotometer.

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3. RESULTS AND DISCUSSION

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3.1 Structural modeling

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In this paper, the software of FDTD Solution is used as analysis for simulating

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different distribution structures of SixOy ARCs with gradient refractive index. The size

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of rectangular model is that the length, width and height are 2.5 µm, 2.5 µm and

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0.1µm respectively. The physical and optical values of silicon oxide materials are

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used as parameters in the simulation. Different structures of SixOy ARCs with gradient

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refractive index are selected to simulate the properties of optical reflection. The

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optimal results can be analyzed to obtain the lowest average reflectance in the

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wavelength range of visible light. The basic cross-section structures of single ARC and double ARCs on the

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substrate have been shown in Fig. 1(A) and Fig. 1(B) respectively. The reflectance of

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incident light of wavelength λ from the surface of substrate covered by a single

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non-absorbing layer is given by Eq. 1:

r12 + r22 + 2r1r2 cos 2θ R= 1 + r12 r22 + 2r1r2 cos 2θ

where r1 and r2 are the individual Fresnel reflectance given by Eq. 2:

r1 =

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n0 − n1 n −n , r2 = 1 s n0 + n1 n1 + ns

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[1]

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[2]

and θ is the phase difference of the single layer, β is refractive angle given by Eq.3:

2θ =

4πn1d1 λ cos β

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And n1 is the refractive index of the thin film, d1 is its thickness, and ns is the

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refractive index of the substrate. The reflectance exists minimum at a quarter

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wavelength where n1d1 = λ0 / 4 and for odd multiples of λ0 / 4 . This minimal value

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is given by Eq. 4:

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R = [( n1 − n0 ns ) ( n1 + n0 ns )]2 2

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And R equals zero if it meets Eq. 5. Since n0 equals one for air medium, the

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refractive index of the film should equal the square root of index of the substrate for

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zero reflectance. The reflective value becomes higher, if it is higher or lower than this

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quarter wavelength value due to variations in the refractive index with wavelength.

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n1 = ( n0 × ns )1 2

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Lower average reflectance can be obtained using double ARCs instead of single

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layer, where the inner layer next to the substrate and the outer layer upon the inner

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layer. The double layer system is a better match between the high refractive index of

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the substrate and the low refractive index of air. The reflectance of light from the

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surface of the structure is given by Eq. 6: r12 + r22 + r32 + r12 r22 r32 + 2r1r2 (1 + r32 ) cos 2θ1 + 2r2 r3 (1 + r12 ) cos 2θ 2 R=

+ 2r1r3 cos 2(θ1 + θ 2 ) + 2r1r22 r3 cos 2(θ1 − θ 2 ) 1 + r12 r22 + r12 r32 + r22 r32 + 2r1r2 (1 + r32 ) cos 2θ1 + 2r2 r3 (1 + r12 ) cos 2θ 2 + 2r1r3 cos 2(θ1 + θ 2 ) + 2r1r22 r3 cos 2(θ1 − θ 2 )

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r1 =

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where r1, r2 and r3 are the individual Fresnel reflectance given by Eq. 7:

n0 − n1 n − ns n − n2 , r2 = 1 , r3 = 2 n0 + n1 n1 + n2 n2 + n s

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where ns is now the index of the substrate, θ1, θ2 are the phase difference and β, γ are

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refractive angle of the double layers given by Eq. 8:

2θ1 =

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The reflectance has either a minimum or a local maximum for quarter wavelength optical films ( n1d1 = n2 d 2 = λ0 / 4 ). This reflectance is given by Eq. 9:

R = [(n12 ns − n0 n22 ) (n12 ns + n0 n22 )]2

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4πn1d1 4πn2 d 2 ,2θ 2 = λ cos β λ cos γ

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2 2 Which R approaches zero if the condition meets n2 / n1 = ns n0 , and

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approaches a local maximum with zero reflectance on either side if the condition

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meets n1n2 = n0 ns . Therefore the average reflectance for double ARCs are lower

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over a broader wavelength range than for a single ARC, because single ARC has only

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minimal value of reflectance [21].

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3.2 Morphology and structure We select ten kinds of samples deposited by different gas ratio of SiH4 to N2O,

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such as 1:0.5, 1:1, 1:2 and 1:3 respectively, to investigate the cross-section

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morphology of thin films. The related experimental and measured parameters of

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samples for single- and double-layer films have been shown in Tab. 2. The characterization of XRD patterns is also displayed in Fig.2. In the picture,

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SixOy and Si phase can be obtained through different peak shift. The SixOy exhibits a

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broad peak at ~28° (sample c in Fig.2). Because SixOy thin films still includes parts of

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amorphous phase, the broad XRD peak at angle ~28° demonstrates amorphous phase.

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When the ratio of SiH4 in mixed gas is increased, a new peak of crystalline silicon

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(c-Si) appears at angle ~30° corresponding to the standard XRD cards for crystalline

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Si (111) (sample a and sample b in Fig.2).

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The cross-section SEM image of sample h has been demonstrated in Fig. 3(A),

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indicating that the SixOy film thickness of sample h is about 112 nm. The double SixOy

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ARCs have been deposited on the glass substrate coating Sn2O3:In transparent

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conductive oxide (ITO), because SixOy layer and glass substrate can be separated

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obviously by ITO layer from SEM image. The atomic ratio of element Si to O (x/y) is

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about 56.59/43.41=1.3, which is characterized by EDS analysis in Fig. 3(B). As a

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result, sample h is proven to be the best condition of SixOy ARC with gradient

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refractive index, which can also be confirmed by optical results experimentally and

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numerically later.

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3.3 Optical properties The refractive index and thickness of SixOy thin films measured by spectroscopic

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index and gas ratio of single SixOy thin films has been shown in Fig. 4. When the ratio

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of SiH4 in mixed gas is increased, the velocity of growth for SixOy thin film becomes

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faster and the value of refractive index for thin film becomes higher, because the

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increment of SiH4 ratio is benefit to enhance the thickness of SixOy thin film in the

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same deposition time. It is found that the gradient refractive index of SixOy ARCs can

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be obtained by adjustment of SiH4 ratio in mixed gas.

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In the structure of ARC on the substrate, the reflection spectra are analyzed by

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FDTD simulation. The reflection spectra are assumed to vary through different

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refractive index of single and double layer thin film. The parameters of refractive

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index and thickness in Tab. 2 are originated from spectroscopic ellipsometer. It is

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proved theoretically that the correlation between reflection spectra and gradient

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refractive index distribution can be obtained from FDTD simulation. Meanwhile the

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reflection spectra of SixOy ARCs with gradient refractive index are deduced from the

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UV-visible spectrophotometer. We obtained reflection spectra by UV-visible spectral

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characterization as illustrated in Fig. 5(A) and FDTD simulation as shown in Fig.

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5(B). The reflection spectra corresponding to sample a, h, i and j have been displayed

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in Fig. 5 respectively.

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When the substrate is silicon material, the optimized value of refractive index for

n0 ns = 3.9 ≈ 2 by Eq.5. To obtain lowest average

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single ARC is about

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reflectance (< 10%), the best ratio of gradient refractive index for double ARCs must

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meet Eq.9. For above reasons, It is found that sample h is optimized double ARCs

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samples. Therefore the reflection spectra simulated by FDTD method in Fig. 5(B)

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accords very well to the experimental result characterized by UV-visible

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spectrophotometer in Fig. 5(A), especially in wavelength range from 500 nm to 600

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nm.

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4. CONCLUSIONS

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In this paper, the single- and double-layer structures of SixOy ARCs with gradient

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refractive index are numerically and experimentally investigated. The FDTD

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simulative model is used to predict the optimal reflection spectra corresponding to

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double ARCs structure of gradient refractive index distribution. The lowest average

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reflectance (< 10%) in broader wavelength range is achieved by optimized gas ratio of

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SiH4 to N2O (outer layer 1:3, inner layer 1:1) in the experiment. The optical

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characterization of thin films with gradient refractive index accords very well to the

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structural modeling in the simulation.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the project funded

by National Science Foundation of China (Grant no. 61404080 and 51202139).

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Table Caption:

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Table 1 Experimental parameters of SixOy:H thin film samples deposited by PECVD system

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Table 2 The refractive index and thickness of SixOy ARC samples for different gas ratio

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Figure Caption:

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Fig. 1 The basic cross-section structures of (A) single ARC on the substrate (B) double ARCs on

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the substrate

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Fig. 2 The characterization of XRD patterns for sample a, b and c

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Fig. 3 (A) The cross-section structure of sample h characterized by SEM (B) The weight and

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atomic percentage of element for sample h characterized by EDS

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Fig. 4 The correlation between refractive index and gas ratio of SiH4 to N2O for single SixOy thin

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films

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Fig. 5 (A) UV-visible reflection spectra and (B) FDTD simulative reflection spectra of sample a, h,

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i and j

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Specific values

Pressure (Torr)

0.4-0.6

Temperature (°C)

200-250

The Ar2 flow of pretreatment (sccm) Frequency (M Hz)

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Parameters of deposited SixOy:H thin film

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13.56

60-80

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RF power (W)

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Refractive index

Thickness (nm)

a b c d

10:5:100 (1:0.5:10) 10:10:100 (1:1:10) 10:20:100 (1:2:10) 8:24:80 (1:3:10) Outer layer: 10:10:100 (1:1:10) Inner layer: 10:5:100 (1:0.5:10) Outer layer: 10:20:100 (1:2:10) Inner layer: 10:5:100 (1:0.5:10) Outer layer: 8:24:80 (1:3:10) Inner layer: 10:20:100 (1:2:10) Outer layer: 8:24:80 (1:3:10) Inner layer: 10:10:100 (1:1:10) Outer layer: 8:24:80 (1:3:10) Inner layer: 10:5:100 (1:0.5:10) Outer layer: 10:20:100 (1:2:10) Inner layer: 10:10:100 (1:1:10)

2.421 2.256 2.013 1.674

95.7 84.8 76.4 67.5

2.287

118.6

g h i

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2.224 1.897 2.064

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2.105

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Sample No.

2.093

107.4 98.5

112.5

105.2 115.7

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>The SixOy thin film is deposited on the substrate as antireflective coating by PECVD >Different reflection spectra with gradient refractive index are obtained by simulation > Reflection spectra with different structure are measured by UV-visible spectrometer > The characterization of reflective spectra verifies the modeling in FDTD simulation